While MSMA is applied to beetle-infested trees in BC to restrict the spread of an infestation, this study also shows that the pesticide is not entirely effective at killing off mountain pine beetles.

Those bark beetles that do survive in an MSMA-treated tree accumulate arsenic. Live adult beetles found in the summer after trees were squirted with MSMA had up to 140 micrograms of arsenic per gram dry weight compared with 0.19 in beetles from nearby untreated trees. Mountain pine beetle larvae have even greater tolerances and many survived with arsenic concentrations as high as 327 micrograms.

Extensively shredded bark gave evidence that woodpeckers were after beetles in pesticide-treated trees, although the birds tended to avoid trees containing beetles with the highest arsenic concentrations. Blood from 79 percent of three-toed and hairy woodpeckers and from mountain chickadees that nested within a kilometre of trees treated in the previous two years contained higher than normal arsenic levels.

Over half the birds had above 0.14 micrograms arsenic per g dry weight, more than double the background concentration of arsenic. Surprisingly though, a red-naped sapsucker had the highest arsenic level, at 3.73 micrograms per g dry weight. Since sapsuckers mainly feed on tree sap rather than beetles, this finding suggests that some birds may be picking up MSMA compounds directly from trees.